The present invention relates to an aluminum sheet embossing roll used for embossing the surface of an aluminum sheet and thereby imparting recessed portions and protruded portions. The present invention also relates to a method of manufacturing lithographic printing plate supports using such a roll.
One known method of manufacturing aluminum supports for printing plates used in lithographic printing (which are referred to hereinafter as “lithographic printing plate supports”) involves using a steel roll that has been imparted with the recessed portions and the protruded portions on the surface by shot-blasting to roll an aluminum sheet and thereby impart recessed portions and protruded portions to the surface of the sheet (JP 60-36196 A (the term “JP XX-XXXXXX A” as used herein means an “unexamined published Japanese patent application”)). Other known methods include the process described in JP 62-25094 A in which rolling is carried out at a rolling reduction of 2 to 20% using a steel roll fabricated by honing (which has an Ra of 0.5 to 1.5 μm and at least 0.6 μm deep of 500/mm2 or more recessed portions and protruded portions), the process described in JP 62-111792 A in which rolling is carried out at a rolling reduction of 2 to 20% using a roll chemically etched or honed to an Ra of 0.5 to 1.5 μm and to a number of recessed portions and protruded portions at least 0.6 μm deep of 500/mm2 or more, and the process described in JP 62-218189 A in which rolling is carried out at a rolling reduction of 2 to 20% using a roll textured to form recessed portions and protruded portions by electrodischarge machining to an average surface roughness Ra of 0.7 to 1.7 μm and a number of recessed portions and protruded portions at least 0.6 μm deep of 500/mm2 or more.
In rolls used for such a metal rolling operation, it is known that when the positions of peaks (protruded portions) on the surface of the roll having the recessed portions and protruded portions (such peak positions are also referred to below as the “roll surface peak height”) are uniform, this helps to increase the life of the roll.
However, because these prior-art rolls employed to roll aluminum sheet for use as lithographic printing plate supports are subjected to blasting such as air blasting or shot blasting in which an abrasive is fired at the surface to roughen it, the resulting peaks on the roll surface are of non-uniform height. Accordingly, it has been difficult to obtain rolls having sufficiently large recessed portions and protruded portions and surface peaks of a sufficiently uniform height, such as are desired for embossing aluminum sheets to be used as lithographic printing plate supports.
Moreover, when these prior-art rolls are employed to manufacture embossed aluminum sheets for use as lithographic printing plate supports, particularly in computer-to-plate applications (commonly abbreviated as “CTP,” this refers to technology in which digitized image data is carried on a highly convergent beam of radiation such as laser light which is scanned over a presensitized plate to expose it, thus enabling the direct production of a lithographic printing plate without relying on the use of lith film), it is difficult to achieve supports having excellent printing characteristics, particularly press life (number of impressions) and sensitivity.
JP 64-8293 A describes a chromium-plated roll for use in such processes as rolling steel sheet. This roll is obtained by subjecting a dull-finished roll as the anode to electrolytic treatment in an electrolyte solution so as to increase the peaks per inch (PPI) on the roll surface by 1 to 50% relative to before electrolysis, then chromium plating the treated roll.
Also known to the art is a chromium-plated metal-rolling roll having a surface roughness Rz which has been lowered by 5 to 20% relative to the initial roughness, either before or after chromium plating (JP 61-202707 A); a chromium-plated roll obtained by the use of, in an etching operation, a chromium plating solution composed of chromium trioxide and sulfuric acid to carry out chromium plating with the roll serving as the anode after the surface roughness Rz of the roll has been lowered by 5 to 20% from the initial roughness (JP 61-201800 A); and a chromium-plated roll obtained by carrying out electrolytic treatment on a bright-finished roll as the anode in a chromium plating solution so as to increase the PPI on the roll surface 1.3 to 15 times relative to the initial value, then administering chromium plating treatment using the roll as the cathode and subsequently polishing the plated roll surface (JP 1-123094 A).
In addition, JP 2001-240994 A discloses a method of manufacturing a chromium-plated roll in which the roll substrate as the anode is subjected to electrolytic treatment in an electrolyte solution. Next, chromium plating is carried out in a chromium plating solution having an iron concentration of less than 5 g/dm3 and using the roll substrate as the cathode by raising the current density from 0 to a level of 25 to 35 A/dm2 over a period of 10 to 30 minutes, maintaining the current density at this level for 2 to 3 minutes, then lowering the current density and holding it at 20 to 30 A/dm2.
In some of these rolls, prior to being chromium plated, the surface of the steel roll is etched by electrolysis to increase the adherence of the chromium plating layer. However, in rolls used for such purposes as rolling steel sheet, regardless of whether such rolls are very smooth rolls for obtaining bright steel sheet or suitably roughened rolls for obtaining dull steel sheet, the chromium-plated surface of the roll is intended for smoothly rolling and finishing cold-rolled steel sheet. The shape required at the surface of the finished product thus differs entirely from that in transfer rolls for embossing aluminum sheet.
Other techniques related to the metal-rolling roll, and the manufacturing method, manufacturing apparatus and plating apparatus for the metal-rolling roll includes techniques described in JP 7-180084 A (plating apparatus), JP 63-99166 A (the apparatus for polishing a roll to a mirror-like finish), JP 8-27594 A (the steel sheet production process and chromium-plated roll for rolling steel sheet), JP 5-65686 A (the method of manufacturing dull rolls for rolling metal), JP 2003-171799 A (the batch-type chromium plating method and apparatus), JP 3-47985 A (the chromium plating process), and JP 2002-47595 A (the chromium plating method and apparatus).